1. Field of the Invention
The present invention is related to the field of Nuclear Magnetic Resonance (NMR) sensing apparatus and methods. More specifically, the present invention is related to NMR well logging apparatus and methods for NMR sensing within earth formations surrounding a wellbore. The present invention also relates to methods for using NMR measurements to determine properties of the earth formations surrounding the wellbore.
2. Description of the Related Art
The description of the present invention and the background thereof are approached in the context of well logging because well logging is a known application of NMR measurement techniques. It is to be explicitly understood that the present invention is not limited to the field of well logging.
NMR well logging instruments can be used for determining properties of earth formations including the fractional volume of pore space and the fractional volume of mobile fluid filling the pore spaces of the earth formations. Methods of using NMR measurements for determining the fractional volume of pore space and the fractional volume of mobile fluid are described, for example, in Spin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination, M. N. Miller et al, Society of Petroleum Engineers paper no. 20561, Richardson, Tex., 1990.
NMR oil well logging instruments known in the art typically make measurements corresponding to an amount of time for hydrogen nuclei present in the earth formations to substantially realign their spin axes, and consequently their bulk magnetization, with an applied magnetic field. The applied magnetic field is typically provided by a permanent magnet disposed in the NMR well logging instrument. The spin axes of hydrogen nuclei in the earth formation, in the aggregate, align with the magnetic field applied by the magnet.
The NMR instrument also typically includes an antenna, positioned near the magnet and shaped so that a pulse of radio frequency (RF) power conducted through the antenna induces an RF magnetic field in the earth formation. The RF magnetic field is generally orthogonal to the field applied by the magnet. This RF pulse, typically called a 90 degree pulse, has a duration and amplitude predetermined so that the spin axes of the hydrogen nuclei generally align themselves perpendicularly both to the orthogonal magnetic field induced by the RF pulse and to the magnetic field applied by the magnet. After the 90 degree pulse ends, the nuclear magnetic moments of the hydrogen nuclei gradually "relax" or return to their original alignment with the magnet's field. The amount of time taken for this relaxation, referred to as T1, is related to petrophysical properties of interest of the earth formation.
After the 90 degree pulse ends, the antenna is typically electrically connected to a receiver, which detects and measures voltages induced in the antenna by precessional rotation of the spin axes of the hydrogen nuclei. The precessional rotation generates RF energy at a frequency proportional to the strength of the magnetic field applied by the magnet, this frequency being referred to as the Larmor frequency. The constant of proportionality for the Larmor frequency is known as the gyromagnetic ratio (y.sub.o). The gyromagnetic ratio is unique for each different chemical elemental isotope. The spin axes of the hydrogen nuclei gradually "dephase" because of inhomogeneities in the magnet's field and because of differences in the chemical and magnetic environment within the earth formation. Dephasing results in a rapid decrease in the magnitude of the voltages induced in the antenna. The rapid decrease in the induced voltage is referred to as the free induction decay (FID). The rate of FID is typically referred to by the notation T2*. The FID decay rate consists of a first component, referred to as "true T2", which is due to internal molecular environmental effects, and a second component resulting from microscopic differences in magnetic field gradients and inhomogeneities in the earth formation. The effects of the second component can be substantially removed by a process referred to as spin-echo measurement.
Spin echo measurement can be described as in the following discussion. After a predetermined time period following the FID, another RF pulse is applied to the antenna. This RF pulse has an amplitude and duration predetermined to realign the spin axes of the hydrogen nuclei in the earth formation by an axial rotation of 180 degrees from their immediately previous orientations, and is therefore referred to as a 180 degree pulse. After the end of the 180 degree pulse, hydrogen nuclear axes that were precessing at a slower rate are then positioned so that they are "ahead" of the faster precessing spin axes. The 180 degree reorientation of the nuclear spin axes therefore causes the faster precessing axes to be reoriented "behind" the slower precessing axes. The faster precessing axes then eventually "catch up" to, and come into approximate alignment with, the slower precessing axes after the 180 degree reorientation. As a large number of the spin axes thus become "rephased" with each other, the hydrogen nuclear axial precessions are again are able to induce measurable voltages in the antenna. The voltages induced as a result of the rephasing of the hydrogen nuclear axes with each other after a 180 degree pulse are referred to as a "spin echo".
The spin echo induced voltage is typically smaller than the original voltage generated after cessation of the first RF pulse, because the aggregate nuclear axial alignment, and consequently the bulk magnetization, of the hydrogen nuclei at the time of the spin echo is at least partially realigned with the magnet's field and away from the sensitive axis of the antenna. The spin echo voltage itself decays by FID as the faster precessing nuclear axes quickly "dephase" from the slower precessing nuclear axes.
After another period of time, typically equal to two of the predetermined time periods between the initial 90 degree RF pulse and the 180 degree pulse, another RF pulse of substantially the same amplitude and duration as the 180 degree pulse is applied to the antenna. This subsequent RF pulse causes another 180 degree rotation of the spin axis orientation. This next 180 degree pulse, and the consequent spin axis realignment again causes the slower precessing spin axes to be positioned ahead of the faster precessing spin axes. Eventually another spin echo will occur and induce measurable voltages in the antenna. The induced voltages of this next spin echo will typically be smaller in amplitude than those of the previous spin echo.
Successive 180 degree RF pulses are applied to the antenna to generate successive spin echoes, each one typically having a smaller amplitude than the previous spin echo. The rate at which the peak amplitude of the spin echoes decays is related to petrophysical properties of interest of the earth formations. The number of spin echoes needed to determine the rate of spin echo amplitude decay is related to the properties of the earth formation; in some cases as many as 1,000 spin echoes may be needed to determine the amplitude decay corresponding to the properties of the earth formation which are of interest. The rate at which the peak amplitude of the spin echo measurements decays is directly related to the true T2. True T2 is related to parameters of interest in the earth formation.
One type of NMR well logging apparatus is described, for example in U.S. Pat. No. 4,350,955 issued to Jackson et al. The apparatus disclosed in the Jackson et al '955 patent includes permanent magnets configured to induce a magnetic field in the earth formations which has a toroidal volume of substantially uniform magnetic field strength. A particular drawback to the apparatus disclosed in the Jackson et al '955 patent is that the thickness of the toroidal volume is very small relative to typical rates of axial motion of well logging tools. Well logging tools, in order to be commercially useful, typically must be able to be moved axially through the wellbore at rates not less than ten feet per minute. The length of time needed to make a typical NMR spin-echo measurement set can be as long as several seconds. The NMR logging tool is therefore likely to move a substantial distance during a measurement cycle. Measurements made by the apparatus disclosed in the Jackson et al '955 patent are therefore subject to error as the apparatus is moved during logging operations, because the antenna would no longer be positioned so as to be sensitive to the same toroidal volume which was magnetized at the beginning of any measurement cycle.
Another drawback to the apparatus disclosed in the Jackson et al '955 patent is that it does not eliminate NMR signal originating within the fluid filling the wellbore.
A still further drawback to the apparatus disclosed in the Jackson et al '955 patent is that the toroidally shaped static magnetic field is subject to changes in field strength as the instrument is subjected to changes in ambient temperature and variances in the earth's magnetic field. The antenna in the Jackson et al '955 apparatus is tuned to a single frequency. If the field strength of the static magnetic field in the toroidal volume changes, the antenna may no longer be sensitive to NMR signals originating from within the toroidal volume. Using the apparatus in Jackson et al '955, it is impractical to compensate the frequency of the RF magnetic field for changes in the static magnetic field strength within the toroidal volume.
An apparatus disclosed in U.K. patent application no, 2,141,236 filed by Clow et al and published on Dec. 12, 1984 provides improved signal-to-noise ratio when compared with the apparatus of Jackson et al '955 by including a high magnetic permeability ferrite in the antenna. However, the apparatus disclosed by Clow et al is subject to similar limitations and drawbacks as is the Jackson et al '955 apparatus.
Another NMR well logging apparatus is described, for example in U.S. Pat. No. 4,710,713 issued to Taicher et al. The apparatus disclosed in the Taicher et al '713 patent includes a substantially cylindrical permanent magnet assembly which induces a static magnetic field having substantially uniform field strength within an annular cylindrical volume.
The apparatus disclosed in the Taicher et al '713 patent is subject to several drawbacks. First, because the antenna is located within the strongest part of the magnet's field, when RF electrical pulses are applied to the antenna acoustic waves can be generated in the antenna by an effect known as the "Lorenz force". The antenna returns to its original shape in a series of damped mechanical oscillations in a process referred to as "magnetoacoustic ringing". Ringing can induce large voltages in the antenna which interfere with the measurement of the voltages induced by the NMR spin echoes. Additionally, the magnet is located in the highest strength portion of the RF magnetic field. The magnet can be deformed by magnetostriction. When each RF power pulse ends, the magnet tends to return to its original shape in a series of damped mechanical oscillations, in a process referred to as "magnetostrictive ringing", which as magnetoacoustic ringing, can induce large voltages in the antenna making it difficult to measure the spin echoes.
A further drawback to the apparatus in the Taicher et al '713 patent is that the antenna induces an RF magnetic field in the formations surrounding the tool which decreases in strength as the square of the radial distance from the axis of the magnet. Moreover, a significant portion of the RF energy can be lost in an electrically conductive fluid in the wellbore. Because the signal-to-noise ratio of NMR measurements made in a gradient magnetic field is typically related to the strength of the RF magnetic field, the apparatus disclosed in the Taicher et al '713 can have difficulty obtaining measurements having sufficient signal-to-noise ratio at radial distances which are likely to be outside a zone within the earth formations known as the "invaded" zone. The invaded zone is typically formed by introduction, under differential pressure, of the liquid phase of a fluid called "drilling mud" which is used in the process of drilling the wellbore. The liquid phase displaces native fluids within the pore spaces of the earth formations proximal to the wellbore, making near-wellbore measurements unrepresentative of the native fluid content of the earth formations.
Still another drawback to the apparatus disclosed in Taicher et al '713 is that the antenna length is related to the vertical resolution required by the system designer. Typically, the vertical resolution is preferred to be very short. If the antenna in Taicher et al '713 is not made substantially longer than the diameter of the sensitive volume within the earth formation, the strength of the RF magnetic field can decrease faster than the square of the radial distance from the axis of the antenna. Lines of equal RF magnetic field strength can then become substantially elliptically shaped, which does not match the lines of equal strength of the static magnetic field. This drawback can significantly limit the ability of the apparatus in Taicher et al '713 to make measurements outside the invaded zone.
Another drawback to the apparatus of the Taicher et al '713 patent is that the antenna must be connected to complicated, difficult to build tuning circuitry in order to establish an operating frequency for the RF pulses and to receive the spin-echo emitted energy at that same frequency. It can be desirable to operate the antenna at a plurality of substantially different frequencies in order to measure properties of the earth formation at a plurality of radial distances from the axis of the NMR logging tool. Operating the antenna of the apparatus in the Taicher et al '713 patent at substantially different frequencies can be difficult and expensive, as the antenna cannot be retuned to a different frequency during operation except by connection to different transmitter and receiver circuits each having different tuned electrical characteristics.
Another NMR logging apparatus, known as the Combinable Magnetic Resonance (CMR) logging tool, is described in U.S. Pat. No. 5,055,787 issued to Kleinberg et al. The CMR logging tool includes permanent magnets arranged to induce a magnetic field in the earth formation having substantially zero field gradient within a predetermined sensitive volume. The magnets are arranged in a portion of the tool housing which is typically placed in contact with the wall of the wellbore. The antenna in the CMR tool is positioned in a recess located external to the tool housing, enabling the tool housing to be constructed of high strength material such as steel. A drawback to the CMR tool is that its sensitive volume is only about 0.8 cm away from the tool surface and extends only to about 2.5 cm radially outward from the tool surface. Measurements made by the CMR tool are therefore subject to large error caused by, among other things, roughness in the wall of the wellbore, by deposits of the solid phase of the drilling mud (called "mudcake") onto the wall of the wellbore in any substantial thickness, and by the fluid content of the formation in the invaded zone.
All of the prior art NMR well logging instruments described herein typically have antennas for generating the RF magnetic field and for receiving the NMR signals which are substantially the same length as the axial extent of the static magnetic field. A drawback to prior art NMR apparatus having such antenna dimensions is that measurements made which the instrument is moving are subject to significant error. The first source of error is that the RF magnetic field may be generated in a region different from that which is completely "prepolarized" by the static magnetic field. A second source of error is that the receiving antenna may be sensitive to an axial region which is different from the axial region in which the NMR signal is likely to originate, as the instrument is axially moved during measurement.
Accordingly, it is an object of the present invention to provide an NMR well logging apparatus which provides more accurate measurements while the apparatus is moved axially through the wellbore.
It is another object of the present invention to provide an NMR well logging apparatus which has substantially reduced effects of magnetoacoustic and magnetostrictive ringing.
It is yet another object of the present invention to provide an NMR well logging apparatus which includes selectable RF pulse frequencies to generate NMR measurements at a plurality of preselected radial distances into the earth formation from the axis of the tool.